The present invention relates to a position-fixing system.
A position-fixing system of the prior art is, for example, the Global Positioning System (GPS), a satellite-based navigation system developed in the U.S.A. especially for military applications. The GPS is described in "NAVIGATION, Journal of the Institute of Navigation", GPS Special Issue, Vol. 25, No. 2, Summer 1978, pages 93-106.
In the GPS, about 18 satellites are in orbit. Their orbital period is 12 hours. The satellites are the reference stations for the users of the system, who determine their positions from the positions of the reference stations and the signal transit times to at least four reference stations. A ground control station determines the positions of the satellites' orbits with high accuracy. The orbital data thus determined are transmitted to the satellites. To obtain a fix, a user measures the signal transit times to four reference stations by one-way ranging. In theory, the user position can be determined from three ranges. Since in practice, however, the clocks always differ from the system time, four measurements are made. Four equations are then available for establishing a fix, and it is possible to determine, besides the three unknown coordinates, a fourth unknown, namely the user clock offset from the system time.
To be able to determine his position, the user must precisely known the positions of the reference stations. He determines these positions from the satellites' orbital data transmitted from the ground station to the satellites and from there to the user, and from the time difference between the instants at which the orbital data of the respective satellite were determined by the ground control station and the instant at which the position of the reference station (i.e., the satellite) is determined from the orbital data at the user station. In the GPS it is necessary that all satellites be continuously in operation. The signals transmitted from the satellites are modulated with different codes.